Folliculitis is defined histologically as the presence of inflammatory cells within the wall and ostia of the hair follicle, creating a follicular-based pustule. The actual type of inflammatory cells can vary and may be dependent on the etiology of the folliculitis, the stage at which the biopsy specimen was obtained, or both. The inflammation can be either limited to the superficial aspect of the follicle with primary involvement of the infundibulum or the inflammation can affect both the superficial and deep aspect of the follicle. Deep folliculitis can eventuate from chronic lesions of superficial folliculitis or from lesions that are manipulated, and this may ultimately result in scarring.
Perifolliculitis, on the other hand, is defined as the presence of inflammatory cells in the perifollicular tissues and can involve the adjacent reticular dermis. Folliculitis and perifolliculitis can manifest independently or together as a result of follicular disruption and irritation.
Acne represents a noninfectious form of folliculitis. The follicular inflammation seen in acne occurs as a secondary event as a result of follicular obstruction from abnormal keratinization. In acne, the superficial aspect of the follicle distends and is obstructed by a keratin plug. The sebum fills the follicle, and the normally commensal bacteria (Propionibacterium acnes) produces excess free fatty acids, which trigger follicular inflammation.
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